The present invention relates to apparatuses for oscillating a mass. More particularly, the present invention relates to an apparatus for oscillating a mass against a spring at a natural frequency of the spring-mass system.
Many oscillator systems require that a mass be oscillated at a selected frequency. One such application is related to the exploration of natural resources located below the earth surface. Commonly, exploration includes mapping the subsurface strata in order to ascertain subsurface characteristics. Techniques include using a plurality of acoustical detectors disposed over the surface of the earth or below the surface in existing wells. An acoustical source then generates an acoustical wave. The acoustical detectors receive the reflected and direct acoustical wave from the subsurface strata and that data is used to ascertain the subsurface characteristics.
Although explosives placed in a borehole have been used in the past for generating the acoustical wave, this technique has met with disfavor since the explosive may damage the borehole. Recently, apparatuses have been advanced to establish a resonant standing pressure wave of a desired frequency using the well fluid in the borehole. The standing wave is radiated through the borehole wall into the earth and is eventually received by the acoustical detectors located on the surface or in adjacent boreholes. Although it is generally known that a resonant system yields high energy efficiency because the fluid column is made to oscillate at or near resonance, requiring a relatively small amount of input power to produce a relatively large pressure oscillation, many systems require blocking members to physically isolate the section of the well fluid from the remaining fluid in the borehole. These systems suffer from substantial drawbacks which include the need of moving the blocking members considerable distances down the borehole as well as being able to move the blocking members relative to each other in order to adjust the frequency of the standing pressure wave.